Soybean somatic embryos have attracted attention both as a model of zygotic embryos and as explants for the generation of stable transgenic plants. We have now characterized the maturation of soybean somatic embryos in detail by examining both the accumulation of the major seed storage proteins b-conglycinin and glycinin as well as changes in cellular organization. Protein storage vacuoles and oil bodies, which are the main depositories of seed storage reserves, formed within cells during the maturation of somatic embryos. The seed storage proteins were gradually synthesized and accumulated in the protein storage vacuoles in a manner similar to that apparent in seeds. The a and a Ј subunits of bconglycinin were detected earlier than the b subunit of this protein and glycinin. In addition, The a and a Ј subunits of bconglycinin accumulated in both the cotyledons and the hypocotyl of somatic embryos, whereas the b subunit of bconglycinin and glycinin accumulated only in the cotyledons. These temporal and spatial characteristics of storage protein production in maturating somatic embryos are similar to those in developing seeds, although the maturation of somatic embryos ceases prematurely without attainment of the final stages of development. Our findings suggest that somatic embryos are suitable for verification of seed-specific traits such as the biosynthesis of seed components.
Key words: Seed development, somatic embryogenesis, soybean (Glycine max [L.] Merr.), storage protein.Plant Biotechnology 26, 543-550 (2009) Original Paper a Present address: National Institute of Crop Science, Tsukuba, Ibaraki 305-8518, Japan. Abbreviations: BSA, bovine serum albumin; PAGE, polyacrylamide gel electrophoresis; PBS, phosphate-buffered saline; PCR, polymerase chain reaction; PSV, protein storage vacuole; RT, reverse transcription; a-TIP, a-tonoplast intrinsic protein. This article can be found at http://www.jspcmb.jp/ Mcmullen 1991; Khalafalla et al. 2005). Transgenic immature somatic embryos can also be maintained in culture, and homogeneous masses of such embryos can be readily and repeatedly subjected to induction of differentiation and maturation. Somatic embryos have therefore been used to screen for transgene expression and to examine transgene effects before recovery of whole plants (Cahoon et al. 1999;Cahoon et al. 2000;Cahoon et al. 2002;Herman et al. 2003;Chen et al. 2006;Kajikawa et al. 2008). In addition to the use of somatic embryos as a provisional screening system, we hypothesized that such embryos could be substituted for seeds for validation of gene function in the synthesis of seed components. Mature somatic embryos of soybean have been found to contain seed storage proteins (Komatsuda et al. 1992;Stejskal and Griga 1995), and the fatty acid composition of such embryos is similar to that of seeds (Shoemaker and Hammond 1988;Dahmer et al. 1991). However, relatively little information has been available regarding common features of somatic embryos and seeds.We have now investigated in detail the...